1. Field of the Invention
The present invention relates generally to printing and copying devices, and more particularly relates to electrophotographic printers and copiers utilizing magnetic brush structures to handle a developer material and deposit toner therefrom onto an organic or inorganic photoconductor drum.
2. Description of Related Art
For many years xerographic development processes were utilized in printers and copiers for the development of latent images borne on a photoconductive media. In modern electrophotographic copiers and printers, however, the conventional cascade xerographic process is seldom used, primarily due to the undesirably large size of its necessary components and its relatively low process speed. Instead, a development process commonly referred to as "magnetic brush" development is now widely used in place of the cascade xerographic development process.
In a conventional magnetic brush developer system utilized in an electrophotographic printer or copier, a magnetic roller is rotated within a sump structure in which a predetermined quantity of dry developer mix is disposed, the developer mix consisting of a magnetically attractable carrier material and a dry toner material removably adhered thereto. The rotating magnetic roller attracts a quantity of the developer and magnetically adheres it to its outer side surface, with the carrier portion of the attracted developer quantity being externally coated with toner and projecting radially outwardly from the rotating roller in bristle-like fashion, thus giving the side surface of the roller the appearance of a "brush".
The magnetically adhered quantity of developer is then rotated past a metering blade which "trims" the radially projecting developer material bristles to a predetermined, generally uniform length. The trimmed developer material rotationally exiting the metering blade is then rotated into close adjacency with a side surface portion of a rotating organic photoconductor drum. Previous to being brought into adjacency with the trimmed developer material on the magnetic roller, negatively charged "background" areas, and discharged "image" areas (corresponding to the printed indicia to be transferred to paper stock operatively fed through the printer or copier), are formed on the side surface portion of the rotating drum.
Toner from the trimmed developer layer is electrically stripped from its carrier material and forced onto the previously discharged image areas of the drum side surface portion. The toner-covered discharged areas on the drum are then rotated into adjacency with the paper stock being fed through the machine, and the toner from the discharged drum areas is electrically drawn onto the paper stock and thermally cured thereon.
The drum side surface portion from which toner has been transferred to the moving paper stock is then sequentially rotated past a scraper blade which removes residual toner from the drum portion, a discharge lamp device which removes residual electrical charge from the drum portion, a charging device (such as a scorotron) which negatively charges the drum portion, and a discharge device (such as a digitally controlled laser beam) which forms the electrically discharged image areas on the otherwise negatively charged drum portion. The drum side surface portion is then again rotated into adjacency with a trimmed quantity of developer externally carried by the magnetic roller.
After a given portion of the trimmed developer material carried by the rotating magnetic roller has transferred its toner constituent to the rotating drum, the now toner-depleted developer portion remains magnetically adhered to the roller and is rotated back into the developer sump at which time additional non-depleted developer material (i.e., developer containing both toner and carrier material) is magnetically adhered to the depleted developer layer and passed across the previously mentioned metering blade on its way to the side surface of the rotating drum. This conventional developer transfer scheme, used in conjunction with magnetic brush development, is commonly referred to as an "open loop" developer transfer path.
While the use of this open loop developer routing is widely accepted and practiced in the printer/copier art, it is subject to a variety of well known problems, limitations and disadvantages. For example, conventional open-looped magnetic brush development modules used in electrophotographic machines, such as printers and copiers, are quite susceptible to print quality degradation and variation over the operating life of the module. This is due in large part to the unavoidable progressive build-up of depleted developer material on the magnetic roller. As this depleted layer progressively thickens, the ratio of toner to carrier material in the overall developer layer approaching the metering blade also progressively diminishes.
Related to this problem are the problems of image density depletions and surges, and the difficulty of sufficiently meeting instantaneous demands for additional toner such as when graphics or other large image areas are to be printed. Additionally, the conventional open loop method of magnetic brush development tends to undesirably shorten the effective operating life of a given quantity of carrier material and the OPC drum.
It can readily be seen from the foregoing that it would be highly desirable to provide improved magnetic brush development apparatus and methods which would eliminate or at least significantly reduce the aforementioned problems, limitations and disadvantages typically associated with conventional magnetic brush development apparatus and methods of the general type described above. It is accordingly an object of the present invention to provide such improved apparatus and methods.